The present invention relates to a flow rate control valve and a flow rate control system for precisely controlling the flow rate of a target fluid to be measured.
A stepping motor or a differential transformer is used in a driver for actuating a valve body of a flow rate control valve. In a flow rate control valve in which a stepping motor is used in the driver, the stepping motor looses its holding torque for holding the rotating position of the rotary shaft if the power supply to the stepping motor is stopped. As a result, the valve body is moved by the pressure of the fluid to be measured. It follows that, in order to maintain constant the opening degree of the valve hole, it is necessary to continue to supply power to the stepping motor, leading to an increased power consumption. In addition, the heat generated by the power supply causes the temperature of the fluid to be measured to be elevated. Further, since the stepping motor is relatively large in size, it is difficult to miniaturize the apparatus.
What should also be noted is that, since the accuracy (resolution) of the rotating position control of the stepping motor is determined by the number of teeth of the stator, it is necessary for the stepping motor to include a costly stator having a large number of teeth or a large stator in order to improve the accuracy. It follows that the driver using a stepping motor is not adapted for controlling a low flow rate requiring control of a high precision.
In addition, since the stepping motor generates a magnetic field, the electronic devices arranged around the flow rate control valve tend to be adversely affected to cause, for example, malfunction.
The driver using a differential transformer also gives rise to problems similar to those produced by the driver using a stepping motor. In addition, the differential transformer is caused to incur malfunction in the presence of magnetism.